A variety of boilers and like devices have been proposed in the prior art that make use of oxygen transport membranes to separate oxygen from a heated oxygen-containing feed to support combustion of a fuel. The heat produced by such combustion can be indirectly transferred to boiler feed water flowing within steam tubes to raise steam. A central advantage of such boilers and devices is that carbon dioxide produced by the combustion can be sequestered for environmental purposes and for use in other processes.
Oxygen transport membranes are known devices that incorporate a ceramic material that is capable of oxygen ion transport at elevated temperatures. When an oxygen-containing gas is exposed to one side of the membrane, conventionally known as a cathode side, the oxygen is ionized and oxygen ions are transported through the membrane to the opposite side known as the anode side. The oxygen ions react with a fuel species that consumes the oxygen ions. This consumption of oxygen ions creates a partial pressure difference of oxygen between the cathode side and the anode side of the membrane that provides a driving force for the oxygen ion transport. The partial pressure difference can also be created by compressing a feed stream containing the oxygen and/or reducing the pressure on the anode side.
Electrons are made available for oxygen ionization at the cathode side by electrons being lost from the oxygen ions at the anode side. Certain ceramic materials, formed from perovskites, exhibit both oxygen ion and electron conductivity and thus are known as mixed conductors. In such materials, the electrons flow through the material from the anode to the cathode side. Other ceramic materials are ionic conductors and are capable of only ionic transport. Such materials are thus used in combination with an electrically conductive phase for the electron transport or with an external circuit for electrical circuit. A typical example of such an ionic conductor is yttrium stabilized zirconia.
As indicated above, the heat generated by the combustion of the fuel introduced to the anode side of the membrane can be used to generate steam. Membranes utilized in such boilers can be driven under a positive oxygen partial pressure that is produced by combusting a fuel at the anode side of the membrane. For example, in U.S. Pat. No. 6,394,043 a boiler is disclosed in which steam tubes and ceramic membrane elements are interspersed. Fuel is introduced into the device that reacts with oxygen ions that have been transported through the membrane to generate heat to raise steam in boiler feed water flowing within the steam tubes. This type of boiler has been optimized in a paper entitled, “Cost and Feasibility Study on the Praxair Advanced Boiler for the CO2 Capture Project Refinery Scenario”, Switzer et al., Elsevier (2005). In this paper a boiler is illustrated having rows of oxygen transport membrane tubes located within a housing and alternating with steam tubes to superheat saturated steam by combustion of a fuel supported by oxygen separation. The resulting heated and oxygen depleted retentate is used to heat heated boiler feed water and thereby to generate the saturated steam. Such heating takes place within the housing upstream of the oxygen transport membrane tubes. Part of the flue gas is recirculated, mixed with the fuel and also introduced into the housing.
As can be appreciated, it is desirable to recover heat energy from both the heated flue gas stream and the retentate stream for use in preheating the air feed to the oxygen transport membrane device and for heating boiler feed water. In Switzer, the incoming air is heated against flue gas after having passed through a heat exchanger being used to preheat the boiler feed water. Preheated air is passed through a heat exchanger in which the air is further heated by the retentate stream after having been used to generate the saturated steam. Thereafter, the retentate stream flows into another heat exchanger to further heat the boiler feed water.
The boiler, described above and like systems, operates at high temperatures and therefore require that the air be preheated to a temperature of generally about 900° C. Such high temperature operation requires expensive, high temperature heat exchangers that are necessary to recover heat and thereby capture a sufficient thermal efficiency to make the use of such boilers and systems practical. As will be discussed, the present invention provides an inherently efficient process for recovering heat energy and thereby heating the oxygen-containing stream, the boiler feed water stream and the fuel stream that optimizes the use of the heat exchangers to decrease the costs involved in fabricating such boilers.